1. Field of the Invention
The invention relates to the production of methane from carbon monoxide. More particularly, it relates to a methanation process capable of effectively utilizing carbon monoxide-containing gas streams on a continuous cyclic basis.
2. Description of the Prior Art
It has heretofore been proposed to utilize carbon monoxide present in gas streams, particularly in dilute carbon monoxide-containing waste streams, for the production of methane. This process is uniquely suited for the utilization of carbon monoxide in gas streams not previously suitable as feed streams for the commercial production of methane. In the practice of this process, a carbon monoxide-containing gas stream is passed over a suitable disproportionation catalyst under conditions such that the carbon monoxide is decomposed to form carbon dioxide and active surface carbon, designated as C* and deposited on the catalyst, according to the reaction: EQU 2 CO.fwdarw.CO.sub.2 +C* (1)
The carbon dioxide and inert gases present in the feed stream are vented from the surface layer of active surface carbon, which is thereafter converted to methane by contact with steam as follows: EQU 2 C*+2 H.sub.2 O.fwdarw.CH.sub.4 +CO.sub.2 ( 2)
Upon separation from the accompanying CO.sub.2 by conventional means, methane is recovered in the form of a low-cost, relatively pure product, with the carbon values thus recovered being at least about 12.5 percent and up to nearly 25 percent of the carbon present in the carbon monoxide decomposed upon contact with the catalyst.
The decomposition of carbon monoxide over a disproportionation catalyst, in accordance with reaction (1) above is carried out at a reaction temperature of from about 100.degree. C. to about 350.degree. C., preferably at from about 200.degree. C. to about 300.degree. C. for practical commercial operations. The carbon monoxide-containing gas stream is passed over the catalyst for a time sufficient to deposit a surface layer of active surface carbon on the catalyst essentially without the formation of inactive coke thereon. Such inactive coke is not only itself inert under the methanation conditions of the process, but may tend to reduce the capacity of the catalyst to form active surface carbon in subsequent operations of the cyclic, essentially two-step process.
The active surface carbon deposited on the catalyst, following the venting of inert gases therefrom, is contacted with steam or a steam-containing gas stream to convert said active surface carbon to methane and carbon dioxide in accordance with reaction (2) above. Reaction temperatures of from about 100.degree. C. to about 400.degree. C. may be employed, with temperatures of from about 200.degree. C. to about 350.degree. C. being generally preferred.
With respect to this known process, it is understood that said CO disproportionation temperature refers to the average temperature of the reaction bed and that the particular reaction temperature pertaining to any given commercial application will be subject to inevitable variations depending on the type of operation employed, e.g., fixed, tubular or fluidized bed, and on the capability of temperature control equipment employed in such applications. While the disproportionation reaction temperature may exceed the indicated temperature limits on a transitory basis, the economic and technical practicality of commercial applications of the process are enhanced when the preferred temperature limits are observed. At higher temperatures, the suitability of the catalyst for use in the cyclic two-step process is diminished so that the overall efficiency of the process is adversely effected. The cost of the methane is thereby increased at such less favorable operating conditions.
Both reaction steps (1) and (2) are exothermic. If the process is carried out on a cyclic basis, therefore, a steady build up of heat from cycle to cycle is possible, requiring disruption of the operation for cooling to maintain operating conditions with the preferred limits indicated above. Such a disruption is undesirable, of course, and would contribute adversely to the feasibility of employing the process in practical commercial applications. It is highly desirable in the art that processing techniques be developed to enable the cyclic, two-step process to be carried out in such practical commercial applications without disruption resulting from the build up of heat from cycle to cycle because of the exothermic nature of each of the major processing steps of the process.
It is an object of the invention, therefore, to provide an improved process for the production of methane.
It is another object of the invention to provide a process for the low-cost production of methane from carbon monoxide-containing gas streams in practical commercial operations.
It is a further object of the invention to provide a process for the catalytic production of methane from carbon monoxide-containing gas streams on a cyclic, essentially two-step basis without disruption due to the build up heat from cycle to cycle.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.